The French say employees should work 35 hours a week. Brits tend to be at their desks for 37.5 hours. Meanwhile, Americans typically spend 47 hours a week at work.

Now, researchers have found the perfect number: 39 hours, according to a study from the Australian National University.

"Long work hours erode a person's mental and physical health because it leaves less time to eat well and look after themselves properly,” said lead researcher Huong Dinh from the university's research school of population health.

Shortening the number of hours is particularly important for women, the study found. “They spend much more time on care and domestic work," Dinh said.

The study, which looked at a sample of Australians ages from 24 to 65, even advocates women should work 34 hours per week once home-based chores are considered. The healthy work limit for men was up to 47 hours a week because they spend less time on care or domestic work.

"Given the extra demands placed on women, it's impossible for women to work long hours often expected by employers unless they compromise their health," Dinh said.